When building up a radio system, the desired coverage area is to be attained with as low costs as possible. When the locations of the base stations of the system are considered, both the required traffic capacity and the coverage area to be attained are taken into account. The base stations tend to be placed so that the coverage area to be attained is great and the base station is situated in a preferable location for the propagation of radio waves. For this purpose, there are various methods and auxiliary means in radio network planning. Vector maps to which the terrain and building information of a desired area is mapped are generally used as auxiliary means in radio network planning. By means of a vector map, with the help of a computer, coverage areas and parameters associated with the operation of the network can be calculated for various locations of base stations.
The present invention can be applied to cellular radio networks especially in an environment having many buildings where the radio network is realized by means of microcells. Microcelis refer to cells considerably smaller in size than conventional cells in the cellular radio system. Microcells are typically used in the city centres where a lot of traffic capacity is needed.
When path attenuation of a radio wave is calculated in the micro cell environment, the propagation mechanisms are generally divided into diffraction, reflections, scattering and direct wave. When this kind of a deterministic propagation model is used in radio network planning, the problem will be the length of time spent for calculation.
For example, multiple diffraction is calculated slowly because there will be very many different combinations of connections between corners even in simple building maps. The method of double diffraction is generally used in the propagation model calculation where the corners visible to the transmitter are examined. It is tested at each corner which calculation points are in the line of vision of the corner. The diffraction field of this corner is calculated therein. It is further tested which corners are in the line of vision of this corner and these corners and calculation points in the line of vision of the corners are examined. The method is very slow as it has to be calculated at each calculation point if there is visibility to the corner to be diffracted. If there are many calculation points, in grid maps generally (10.sup.6 -10.sup.7), the visibility check of each point to every corner to be diffracted is a very laborious operation.
A corresponding situation also occurs when calculating reflections. A so-called ray-tracing method is used for calculating multiple reflections. Ray-tracing can be carried out in two ways: by means of multiple mirror images or by means of a ray launching method.
The problem with determining multiple reflection routes by means of mirror images is that the number of different wall combinations and mirror images is generally very high and the required calculation is arduous and slow. Similarly, the calculation capacity required for the ray launching method is great and the method is also inaccurate.